Ferromagnetic Spin-1/2 Dimers with Strong Anisotropy in MoCl5

The pentachloride MoCl5 adopts several molecular crystal structures, all comprising isolated Mo2Cl10 units with well-separated Mo–Mo magnetic dimers. Using magnetization measurements, single-crystal X-ray diffraction, and first-principles calculations, we confirm ferromagnetism with strong anisotropy below a Curie temperature of 22 K in α-MoCl5, and report a fifth polymorph, ϵ-MoCl5 , that we find to be ferromagnetic below 14 K. Magnetization measurements indicate unquenched orbital moments antialigned with the spins. This is confirmed by first-principles calculations, which also predict an unusually strong magnetocrystalline anisotropy in α-MoCl5 arising from spin–orbit coupling. An anisotropy field near 80 T is calculated, while a smaller but still substantial anisotropy field exceeding 12 T is realized experimentally. Further, increased anisotropy and Curie temperature are predicted when W is substituted for Mo. Similarly, strong magnetism and anisotropy are predicted for isolated Mo2Cl10 molecules, in...